KABUL—The Afghan president's push to delay a security pact with the U.S. has undermined confidence in Afghanistan's future, testing international support for the country at a crucial moment, the U.S.-led coalition's top commander said.

U.S. Marine Corps Gen. Joseph Dunford warned that President Hamid Karzai's failure to quickly sign the agreement could weaken the Afghan economy, embolden the country's powerful neighbors and ultimately...